1. Field of the Invention
The present invention relates to correlation of signals received by a phased array of transducers, and more particularly, to a signal encoding and decoding system for a phased array of transducers in a nondestructive ultrasonic inspection system.
Ultrasonic methods are in use today for testing for flaws in manufactured parts, wherein a single transducers is scanned back and forth across the area to be inspected. In the aircraft industry, graphite and/or other composite and laminated structures are coming into operational use, and these new composite materials entail additional inspection requirements. These requirements include high speed inspection of a multiplicity of drilled holes, high speed inspection of large areas and of new structural shapes which are difficult to inspect, and new field inspection demands after the aircraft are put into service.
In the pulse-echo mode, the ultrasonic transducer is coupled with the part being inspected and detects defects by scanning around each drilled hole or along the length and width of the formed component. That is, the transducer must physically "paint" the area requiring inspection.
Some multi-transducer approaches are coming into use and some even involve electronic scanning and steering. It is still a problem, however, to determine from the complex return signals received by the array just where the signals came from in the volume of material or space under consideration. In the phased array systems, the return signals will arrive at the transducers from a scattering point with various relative delay times dependent upon their distance from the scattering point. Since these signals overlap somewhat in time, they cannot be put into one single delay line and many separate variable delay lines for a multi-element array are very expensive.
The return signals, which are analog in nature, could each conceivably be converted to digital form before individual time delays were accomplished, but this would require expensive, fast-acting analog-to-digital coverters, one for each transducer.
2. Description of the Prior Art
U.S. Pat. No. 3,261,969 to Routh discloses an autocorrelation system for processing sonar signals, from one transducer input, which uses a dispersive delay line.
U.S. Pat. No. 3,158,830 to Clay, Jr. discloses an acoustic depth measurement system employing a signal correlation means with a rotating delay line, and another embodiment having matched filter means. This prior system correlates the return signal with a replica of the transmitted signal for the purpose of signal to noise improvement. Applicant's present invention does not correlate the received and transmitted signals but cross-correlates many received signals.
U.S. Pat. No. 3,918,024 to Macovski discloses an ultrasonic focused array imaging system employing variable delay lines. A system having a large number of necessarily complicated delay lines as outlined in this prior patent would be impractical. U.S. Pat. No. 4,005,382 to Beaver is somewhat similar.
U.S. Pat. No. 3,005,335 to Erdman is typical of several references which employ chirped waves for the purpose of measuring time delay wherein the chirp signals themselves are actually radiated and received. Such systems do not cross-correlate a plurality of received signals.
U.S. Pat. No. 3,639,695 to Bertheas is an example of radar or sonar systems having dispersive networks for signal compression.
U.S. Pat. No. 3,775,734 to Bealor, Jr. et al discloses a multi-channel sonar system which times the transmission sequence so that all sea floor echoes arrive back simultaneously at the receiving transducer.
U.S. Pat. No. 3,872,715 to Pittaro discloses an ultrasonic single-transducer test circuit using gates which open and close upon receipt of an echo signal.
While it is possible that more pertinent prior art exists, Applicant's search is believed to have been conducted with a conscientious effort to locate and evaluate the most relevant art available at the time, but this statement is not to be construed as a representation that no more pertinent art exists.